The present invention relates to the fabrication of optical elements, particularly to forming fiducials on spherical and aspherical optical elements to assist in fabrication and alignment, and more particularly to embedding fiducials below (3-10 .mu.m) the surface of the optical element and outside the clear aperture thereof, and to a method for embedding the fiducials.
In recent years substantial research and development have been directed to various types of imaging system, particularly in the fabrication and alignment of complex optical components in imaging systems such as the extreme ultraviolet (EUV) lithography system. Fiducials, marks that are put on an optical surface and referenced to a coordinate system, are important for optical fabrication and alignment, particularly when individual optical elements are aspheres. It is desirable to put fiducials on an optical surface at the start of the fabrication process. This is particularly important in the fabrication of aspheres where the equation describing the asphere departure must be located very accurately on the surface of the optic. This location is accomplished with interferometry by recording fiducials along with the interference pattern. The fiducials are then used to calculate true spatial coordinates when they are imaged onto a charge coupled diode (CCD) camera along with the interference pattern. Additionally, fiducials are used to determine the location of optical axis and clear aperture on the optical element.
When fiducials are placed on optics, they are usually placed on the back or edge, not directly on the optical surface. Placing them on the optical surface has the advantage of being able to view and record them simultaneously with the interference pattern and use them during the polishing process to attain the proper surface figure (or shape) at a particular location on the optical surface as measured interferometrically. This is particularly important for the fabrication and polishing of aspheric figures. However, placing fiducials on the optical surface before the polishing process is completed presents two problems: 1) the fiducials will be polished off during fabrication; and 2) the fiducials will interfere with the polishing process by generating particulates that can scratch the surface of the optical element.
This problem has been resolved by the present invention, whereby the fiducials are embedded slightly below (3-10 .mu.m) the surface of the optic and outside the clear aperture of the optic. This is accomplished by a lithographic process. Any subsequent polishing of the optical surface does not disturb the fiducials; they become a permanent marking on the optical surface. The embedding process is carried out such that it eliminates an edge around the fiducials which can chip, causing sleeks and scratches on the surface of the optic.